1. Field of the Invention
The present invention relates to an image pickup apparatus. More specifically, the present invention relates to an image pickup apparatus that uses a solid-state image sensor and a zoom lens.
2. Description of the Related Art
The functions of an image pickup apparatus that uses a solid-state image sensor are continuously being improved. For example, the functions of a camera, such as a video camera, a digital still camera, a broadcast camera, or a monitoring camera, have increased substantially in recent years, while the total size of such an apparatus has become smaller and smaller.
At the same time, market forces demand that newly designed image pickup apparatuses be small and highly efficient. To that end, an optical system (zoom lens) used in an image pickup apparatus like a camera is designed with a small-size zoom lens having a wide angle of view (photographic angle of view) and a high zoom ratio, so that the total size of the optical system is kept compact and small.
As a zoom lens having a wide angle of view and a high zoom ratio, a positive-lead type zoom lens has been used. A known positive-lead type zoom lens includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a rear lens unit including one or more lenses.
In order to achieve a positive-lead type zoom lens having a wide angle of view in a small-size optical system while preventing a large-diameter front lens, it may be useful to move the first lens unit during zooming. However, if the first lens unit is tilted while moving during zooming, a photographed image may be blurred.
In addition, if a wide-angle converter is installed around the first lens unit, the first lens unit and the wide-angle converter may interfere with each other during zooming. In order to solve this problem, a conventional zoom lens moves an image sensor when the first lens unit is fixed during zooming.
In a zoom lens discussed in Japanese Patent Application Laid-Open No. 63-68807, one lens unit and an image sensor are moved during zooming. In other words, the image sensor is moved according to an image plane that varies during variable magnification. A zoom lens discussed in U.S. Pat. No. 7,573,647 moves the image sensor, and the zoom lens includes a moving lens unit that corrects variation on the image plane. With the configuration like this, the zoom lens discussed in U.S. Pat. No. 7,573,647, in which the first lens unit is fixed during zooming, can achieve substantially the same effect as that implemented by a zoom lens whose first lens unit is moved during zooming.
In addition, U.S. Pat. No. 7,206,137 discusses a small-size positive-lead type zoom lens having a high zoom ratio. In this case, the small-size positive-lead type zoom lens is constituted by four lens units including, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. Furthermore, a zoom lens discussed in U.S. Pat. No. 7,177,092 is constituted by five lens units including, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power.
Generally, in order to achieve a zoom lens of a small total size while maintaining a high zoom ratio of a predetermined level, it may be useful to increase the refractive power (optical power=an inverse of the focal length) of each lens unit included in the zoom lens. However, in a zoom lens whose refractive power of each lens unit is high, aberrations may greatly vary during zooming. Accordingly, it becomes difficult to achieve a high optical performance for the entire variable magnification (zooming) range.
In the above-described four-unit zoom lens and the five-unit zoom lens, in order to achieve a high optical performance while maintaining a wide angle of view and a high zoom ratio even in a small-size lens system, it may be useful to move a plurality of lens units and an image sensor during zooming. However, it is difficult to achieve a zoom lens having a wide angle of view, a high zoom ratio, and a high optical performance at the same time merely by moving a plurality of lens units and an image sensor during zooming. In other words, it becomes particularly significant to appropriately set the refractive power of each lens unit and determine which lens unit to move during zooming.
More specifically, it is significant to appropriately set conditions for moving the third lens unit during zooming. Unless the configuration described above is appropriately set, it becomes difficult to achieve a zoom lens whose total size is small and having a wide angle of view, a high zoom ratio, and a high optical performance.